Authors observed that the nitrate nitrogen (NO 3 -N) concentration is approximately 30 mg/L and that the average pH value is 4.3 in a small river in Shizuoka Prefecture, Japan. Further, when there is heavy rain, the pH value of the river water decreases to below 3.5 at times. There is a small irrigation reservoir in the watershed. The reservoir receives this water and mixed with other river water, the pH in the reservoir reaches below 5.0 at times, making it impossible for fauna to survive there. This water is also used for paddy irrigation, however no damage to rice production was reported. The authors clarified the fact that the low pH and high NO 3 -N concentrations were brought about by the overuse of nitrogen fertilizers in tea fields. Further, the authors proposed a model, which can be used to estimate the pH value and the NO 3 -N concentration.